José A. Martínez

Transient tachycardia- and bradycardia-dependent left anterior and left posterior hemiblocks. Effects of isoproterenol

Tachycardia- and bradycardia-dependent, left anterior and left posterior hemiblocks as transient phenomena were registered in two patients spontaneously, and especially as a consequence of isoproterenol infusion. A chronic trifascicular type of A-V block was present in the first case, whereas in the second case a bradycardia-dependent left posterior hemiblock was registered during an acute myocardial infarction. In the first patient the isoproterenol effects were: (1) a shortening of the refractoriness and an increase of the conduction velocity in the injured fascicle, (2) an increase in the slope of phase-4 depolarization on the left posterior fascicle, and (3) a presumably shifting toward zero of threshold potential on the left anterior fascicle. Isoproterenol effects disappeared from 30 to 40 min after it was discontinued. In the second case the bradycardia-dependent left posterior hemiblock was registered during very fast heart rates (150 beats min). This finding supports the view that enhanced phase-4 depolarization is the main factor in the development of bradycardia-dependent intraventricular blocks in the course of acute myocardial ischemia.